You are allowed to sit in reserved seats that are not occupied. If you speak to a conductor on a busy train they will tell you to do this. It's usually obvious a few minutes after departure that the person who made the reservation is not coming. But the problem is there is no sign that tells you to do this.
Where is the most comfortable place to sit on a train?
For the smoothest ride, get a seat equidistant between the trucks (wheels). This way a hard bump will be only half as bad. If you want to watch the train ahead going through curves, get a window seat near the rear of the coach.
How to save money on seat selection?
Booking your flight early can make a significant difference when it comes to seat selection. Airlines occasionally provide a few tickets at a reduced price, but they are limited. Book early to for avoiding extra charges for choosing seats.